1. Field of the Invention
The present invention relates to a semiconductor device using a stack having a Heusler alloy, e.g., a spin MOS field effect transistor using the stack having a Heusler alloy as the source/drain, magnetic memory, and tunneling magnetoresistive head (TMR head) using the stack.
2. Description of the Related Art
Recently, a magnetic memory (magnetic random access memory [MRAM]) using a tunneling magnetoresistive (TMR) effect element having a sandwiched structure including a ferromagnetic material/insulator/ferromagnetic material as a memory element has been proposed. This device is used as a memory by fixing spins in one ferromagnetic material layer (a pinned magnetic layer or a fixed layer), and controlling those in the other ferromagnetic material layer (a free magnetic layer), thereby changing the resistance between the two layers in the sandwiched structure. The resistance is low when the spins in the pinned magnetic layer and free magnetic layer are parallel, and high when they are antiparallel. The magnetoresistive change ratio (TMR ratio) as an index of this spin efficiency was a few 10% at room temperature a few years ago, but has reached 500% in recent years. This increases the possibility of the device not only as an MRAM but also as various spin devices. As an example, a spin MOS field effect transistor (spin MOSFET) has been proposed. The spin MOSFET is obtained by adding the degree of freedom of spins to carriers by combining a ferromagnetic material with the conventional MOSFET.
It is important to increase the TMR ratio in order to implement a high-efficiency magnetic memory or spin MOSFET. To this end, it is necessary to use a ferromagnetic material having a high spin polarization ratio (P). When a half-metallic material in which P=100% is used, the TMR ratio is presumably infinite from the Julliere's law. Candidates of a room-temperature, half-metal material are CrO2, Fe3O4, and a Heusler alloy. Recently, Co-based Heusler alloys have achieved high TMR ratios, so spin devices using these alloys are expected. A Heusler alloy is a general term for intermetallic compounds having a chemical composition indicated by X2YZ where X is a Co-, Ni-, or Cu-based transition metal element or noble metal element in the periodic table, Y is an Mn-, V-, or Ti-based transition metal, and Z is a main group element of groups III to V.
To achieve a high TMR ratio by using a Heusler alloy, however, regular epitaxial growth of the crystal structure is indispensable when forming a stacked structure (e.g., N. Tezuka, et al., J. Appl. Phys. 99(2006)08T314), and highly crystallized Heusler alloys are necessary in the spin device. Especially when using a Heusler alloy in the spin MOSFET, a technique of epitaxially growing a Heusler alloy on a semiconductor layer is indispensable.
Also, as prior art relevant to the present invention, a technique capable of forming a (001)-oriented MgO layer on an amorphous layer is disclosed (e.g., S. Yuasa, et al., Appl. Phys. Lett. 87(2005)242503).